1) Field of the Invention
The present invention relates to superplastically formed structural assemblies and, more particularly, to the superplastic forming of an assembly using a friction welded preform.
2) Description of Related Art
Under certain conditions, some materials can be plastically deformed without rupture well beyond their normal limits. This property, called superplasticity, is exhibited by certain metals and alloys within limited ranges of temperature and strain rate. For example, titanium and its alloys are superplastic in the temperature range from about 1450-1850° F. (785-1010° C.). Superplastic forming (SPF) is a technique for expanding or stretching metal that relies on superplasticity. Expansion can and often does exceed 100% and often exceeds 1000%. During one typical superplastic forming process, a preform or blank is placed into a shaping die and heated to a sufficiently high temperature within the superplasticity range of the material to soften the material. Pressurized heated gas is then injected against the material, thereby causing the preform to be urged against the die. Alternatively, the preform can be urged into a desired shape using other methods, such as by using a die. Such superplastic forming methods are further described in U.S. Pat. Nos. 4,117,970; 5,410,132; 5,420,400; 5,700,995; 5,705,794; 5,914,064; 6,337,471; 6,537,682; and 6,568,582, and U.S. application Ser. No. 10/781,419, each of which is incorporated by reference.
Superplastic forming can be performed with certain materials that exhibit superplastic properties within limited ranges of temperature and strain rate. Thus, a preform or blank that is to be superplastic formed must typically be made of a material that is capable of undergoing superplastic forming. However, even if the material is made of a superplastically formable material, cracks or other defects can occur during forming in portions of the preform where the forming characteristics have been compromised. For example, fusion weld joints can become rough or thickened when subjected to superplastic forming and, therefore, any such joints that are subjected to superplastic forming may require post-forming hand working to finish the surface. This adds cost and time to the manufacturing process.
During one conventional superplastic forming operation, a pack is formed of two or more titanium sheets that are stacked and joined by diffusion bonding, e.g., about a periphery of the sheets. With the pack heated, pressurized gas can be injected between the sheets to inflate the pack and thereby superplastically form the sheets. While this operation has proven effective for forming titanium packs, other materials cannot be easily formed in this manner. For example, aluminum does not have the same diffusion bonding characteristics as titanium, and cannot easily be bonded sufficiently for superplastic forming. Further, conventional weld joints formed through the aluminum sheets are typically not capable of undergoing extensive superplastic forming and therefore can fail during the forming operation, e.g., by delamination of the two sheets.
Thus, there exists a need for an improved preform and associated method for superplastically forming structural assemblies. The method should be compatible with aluminum, and should be capable of sufficiently bonding members for superplastic forming.